A subsurface safety valve (“safety valve”) is typically installed in well tubing. Normally, the well is drilled and cased. The casing is cemented and the formation is perforated through the casing. A production string, having a packer is run in and the well is put into production through the tubing. The safety valve is in the production string and is normally operated from the surface through one or more control lines that are run in the annular space between the tubing and the casing and above the packer. The pressure applied to the control line drives a piston against an operating spring. The piston is linked to a flow tube such that applied pressure drives the piston and the flow tube against a flapper to rotate it 90° to open the safety valves. When control line pressure is removed the power or closure spring drives the flow tube in the reverse direction and a spring on the flapper rotates it in the reverse direction against a seat such that flow from below the valve in an uphole direction stops. It should be noted that in these types of completions, the cement does not pass through the valve when the casing is cemented because the production string is run in after the casing is cemented.
Of late, a new type of well system has been used where the casing is not used. Instead, a smaller hole is drilled and production tubing is inserted with a safety valve and the production tubing is cemented into the borehole. In such application the safety valve has to pass cement through the flow tube. The new issue confronting safety valve manufacturers in dealing with this type of application is how to maintain the integrity of the flapper mechanism during the cementing process.
FIG. 1 illustrates a previous unsuccessful attempt to insure the integrity of the safety valve flapper mechanism for operation after cementing through the safety valve. Safety valve 10 has a flapper 12 pinned at 14 for rotation between the open and closed positions, as shown by arrow 16. FIG. 1 is a split view illustrating the open position on top and the closed position at the bottom. The flow tube 18 is actuated by piston 20 that happens to be a rod piston. A control line (not shown) is connected at connection 22 to urge piston 20 down against the opposing force from power spring 24. Thus, a pressure buildup in cavity 26 opens the safety valve 10 by pushing the flapper 12 back behind the translated flow tube. Located in a groove 28 is a resilient o-ring 30 for sealing between the flow tube 18 and the body 32. The thinking behind the addition of the o-ring 30 was to prevent cement pumped through the flow tube 18 from getting in behind it and into the spring cavity 34 and fouling future operation of the safety valve 10. FIG. 1 shows that at the lower end 36 of the flow tube 18 there was a gap 38 to the body 32. This gap was put there for a specific reason. Since the diameter of the flow tube was greater near its upper end 40 than the lower end 36, allowing the gap 36 to be sealed closed could have put a net unbalanced force on the flow tube 18 from internal pressure in passage 42. The problem was, that gap 38 also allowed cement into cavity 44 and that fouled the working of the flapper 12.
Accordingly what was needed was a design that could effectively isolate the flapper during cementing so that the safety valve could function reliably thereafter. At the same time the design needed to be configured so that the flow tube would not become trapped in the valve open position due to sealing off the flow tube in the valve body at its upper and lower ends during the cementing operation. Additionally, the present invention improves on the use of a resilient o-ring seal and substitutes improved sealing around the flow tube for greater reliability. These and other aspects of the present invention can be readily appreciated by those skilled in the art from a review of the description of the preferred embodiment and the claims, which appear below.